This invention relates to storage-stable solid isocyanate compositions that are substantially crystalline at room temperature comprising reaction products of organic isocyanates and substoichiometric quantities of 2-methyl-1,3-propanediol.
It has generally been asserted that diisocyanates that are liquid at room temperature (i.e., approximately 25.degree. C.) have advantages over solid diisocyanates, at least in part because liquid isocyanates are said to be easier to handle and mix than are solid diisocyanates. In particular, the known solid diisocyanates melt so nearly at room temperature that handling is cumbersome or melt at such high temperatures that they are unsuitable for the normally used reaction techniques. As a result, much effort has been made to convert solid diisocyanates to liquid derivatives and to convert liquid diisocyanates to more easily and safely handled liquid derivatives.
The most important commercially available diisocyanates that are solid at room temperature are diphenylmethane-4,4'-diisocyanate and its 2,4'-isomer, which melt at 39.degree. C. and 34.5.degree. C., respectively. Numerous patent references relating to the preparation of liquid diphenylmethane diisocyanates have issued. Some of these approaches include the reaction of the diisocyanate with N,N-di(2-hydroxypropyl)aniline in the presence of phosphoric acid (U.S. Pat. No. 3,394,165); the introduction of carbodiimide groups into the isocyanate (U.S. Pat. Nos. 4,177,205, 4,154,752. 4,143,063, 4,088,665, 4,072,712, 4,014,935, 3,641,093, 3,640,966, and 3,152,162); the reaction of the isocyanate with (i) polyoxyethylene glycols (U.S. Pat. Nos. 4,115,428 and 4,055,548), (ii) propylene and other alkylene glycols (U.S. Pat. Nos. 4,229,347, 4,118,411, 3,892,691, and 3,883,571), (iii) N,N'-disubstituted thioureas (U.S. Pat. No. 3,674,828), (iv) N-substituted ethanolamine (U.S. Pat. No. 4,332,742), and (v) low molecular weight polyether polyols (U.S. Pat. No. 4,102,833); the addition of an organosilicone (U.S. Pat. No. 3,585,230); the addition of a dibenzoate (U.S. Pat. No. 3,701,796); and the combination of partial carbodiimidization with a reaction product of the isocyanate and a diol (U.S. Pat. No. 4,031,026).
Other methods have been used to liquefy diphenylmethane-4,4'-diisocyanate, as well as mixtures of the diphenylmethane-4,4'-diisocyanate with small amounts of the 2,4'-isomer. For example, U.S. Pat. No. 3,644,457 discloses the reaction of diphenylmethane diisocyanate with substoichiometric quantities of certain branched aliphatic diols, such as 1,3-propanediols having at least two alkyl substituents or poly-1,2-propylene ether glycols. Because only 0.1-0.3 molar quantities of the diols are used, the resultant liquid product consists of unreacted diphenylmethane diisocyanate and various adducts of the diols with the isocyanate. The freezing point depression of the isocyanate caused by the presence of the adducts, and thus the viscosity of the resultant product, can be varied by adjusting the number of adducts. In the absence of alkyl substitution, however, aliphatic diols form solids or gels when reacted with excess diphenylmethane diisocyanate. While liquified products made according to this patent have met with commercial success, they still suffer from a serious drawback. Specifically, these products solidify at temperatures near room temperature. As a result, these materials must be kept warm during transport and storage or else must be melted before use, particularly in cold climates. At the same time, the products are too easily melted to be stored as solids, particularly in warm climates.
Other attempts to form liquid diphenylmethane diisocyanates have been described, for example, in U.S. Pat. Nos. 3,449,256, 3,394,164, and 3,384,653, and are based on the addition of a trihydrocarbyl phosphate or small amounts of phosphoric acid. The storage stability of products prepared according to these patents is quite good around room temperature, but as the temperature decreases, such materials tend to solidify.
Conventionally used diisocyanates that are liquid at room temperature, such as toluene diisocyanate or hexamethylene diisocyanate, are generally physiologically harmful because of their high vapor pressure. As a result, such diisocyanates can be used only if certain safety precautions are observed. Consequently, various attempts have been made to apply certain techniques to reduce the physiological effects of isocyanates that are liquid at room temperature. Such techniques, however, usually produce diisocyanates having higher molecular weight or isocyanates of higher valency (i.e., tri- or polyisocyanates) or isocyanates having a combination of such properties.
U.S. Pat. No. 3,457,200 discloses the use of reaction products of toluene diisocyanate with certain low molecular weight diols in the preparation of flexible polyurethane foams. Suitable diols included certain branched aliphatic diols, some of which are similar to diols described in U.S. Pat. No. 3,644,457 for use with diphenylmethane diisocyanates. Not all such diols were found suitable, however, because the resultant products were solids having melting points significantly above room temperature. Moreover, because of reactions between the isocyanate and the urethane hydrogens, even the useful reaction products were said not to be sufficiently storage stable. As a result, the diisocyanate-diol reaction products must be stabilized by reaction with benzoyl chloride or other stabilizing agents.
It is thus apparent that the search for commercially acceptable liquid diphenylmethyl diisocyanates is continuing. Nevertheless, solid diisocyanates having appropriate physical and chemical properties would have certain advantages. For example, a solid diisocyanate that would melt at a temperature in the range normally used to prepare polyurethanes products, for example, about 95.degree.-160.degree. C., could be added to a foamable mixture where it would melt in situ or could be preheated to the reaction temperature before being added to the foamable mixture. It would be particularly advantageous to use a sharply melting crystalline solid diisocyanate that could be ground into a fine crystal or powder for ease in handling and mixing.
It was, therefore, an object of this invention to obtain improved storage-stable organic diisocyanate compositions that are solid even at temperatures higher than room temperature. It has now surprisingly been found that the reaction of certain diisocyanates and higher order isocyanates with 2-methyl-1,3-propanediol produces storage-stable isocyanate compositions that are sharply melting, substantially crystalline solids at room temperature and can be ground into fine crystals or powders. The compositions of the invention have been found eminently suitable for the preparation of polyurethane foams, elastomers, and other polyurethane products.